A power save mode is primarily employed by wireless communications devices, such as mobile phones, to preserve battery life during the time that the mobile phone is turned on but not actively transmitting or receiving packets for some specified period of time. Mobile phones are typically configured to automatically transition to the power save mode if they detect the cessation of traffic to and from their transceiver for some predetermined period of time. If a mobile phone has been configured such that the power save mode is enabled, it will periodically transition from a higher powered state, or a state in which the transceiver is turned on for instance, to a lower powered state, or a state in which the transceiver is turned off for instance, during periods of inactivity. In this lower powered state, the mobile phone will periodically “wake up” or transition to a higher powered state for a short time in order to receive packets of information from the access point with which it is currently associated and then transition back to the lower powered state. Generally, an access point will buffer one or more frames of information addressed to a particular mobile phone that is power save mode enabled and which is in a lower powered state until the phone transitions to a higher powered state, at which time the buffer frames could be transmitted to the mobile phone. The process by which a mobile phone is configured to be power save mode capable and the protocol that a wireless LAN follows for managing the buffering and transmission of frames of information is specified by the IEEE 802.11 standard.
While the power save capability specified by 802.11 addresses the battery life issue, it does not address quality of service issues. To this end, the IEEE 802.11e standard which extends 802.11 into the area of quality of service was developed. Generally, the 802.11e standard specifies formats and protocols to use at an access point and at a mobile phone that enables the prioritized delivery of frames of information according to a user priority or access category assigned to the frames as they are generated by the mobile phone. Specifically with respect to 802.11e standard, there are two different power save modes that have been specified for use to deliver frames of information labeled for priority delivery. One mode allows for the scheduled delivery of traffic by the access point to a phone and is referred to as Scheduled Automatic Power Save Delivery (S-APSD) and a second allows for the unscheduled delivery of traffic by the access point to a phone and is referred to as Unscheduled Automatic Power Save Delivery or U-APSD. These scheduled or unscheduled modes are defined from the perspective of the AP and should be used when it is desirable to provide quality of service (QoS) functionality in conjunction with extended battery life. It is recommended that the S-APSD mode be employed by devices using polled access and that the U-APSD mode be employed by devices using contention-based access. Basically, APSD was developed to add QoS functionality and a more efficient power save mode to the Standard.
Access points which service mobile phones that are configured to operate in the U-APSD mode will buffer frames of audio and other information destined for these phones until they receive a signal or a trigger message from the mobile phone that results in the access point transmitting some or all of the buffered frames to the mobile phone. Trigger frames transmitted by mobile phones when in the U-APSD mode can collide with frames transmitted by the AP, or the trigger frames transmitted by other mobile phones associated with the same access point causing the phone to initiate a back-off procedure and result in delay being injected into the transmission of voice information. This delay has the effect of reducing the quality or capacity of the communications sessions. Such collisions between trigger frames tend to counteract the benefits realized as the result of configuring a mobile phone to operate in the U-APSD mode.
It is one objective of our invention to enable a mobile phone to operate in the U-APSD mode to periodically transmit a trigger message without the possibility of collision with a trigger message transmitted by another mobile phone also configured to operate in the U-APSD mode.
Further, it is another objective of our invention to accomplish this avoidance of collisions without the need for or use of any additional signaling to coordinate between the phones. Such additional signaling would consume more bandwidth and thus reduce the effectiveness of the solution.
We have achieved the above mentioned objectives of our invention by configuring a mobile phone to sense the wireless medium in order to detect the end of another mobile phones unscheduled service period and then to send a trigger message after waiting less than a time interval specified for any other mobile phone associated with the same access point to begin arbitrating for the medium.